1. Field of the Invention
This invention relates to an optical system by which a front object and an approximately lateral object can be simultaneously observed.
2. Description of the Related Art
Optical systems by which a front object and an approximately lateral object can be simultaneously observed have been known up to now. Such optical systems include an optical system which is formed in such a way that, after light from the approximately-lateral-object side is reflected two times on the inside, the light emerges to the image side, and the optical system have been publicly known (for example, refer to Japanese Patent Kokai No. 2008-309895). Besides, in this case, the range which the above term, “approximately lateral”, defines includes not only the lateral side of the optical system itself but also the diagonally front and diagonally backward sides of the optical system.